This application claims the priority of Canadian Patent Application No. 2,315,997 filed Aug. 15, 2000.
The present invention relates to optical waveplates useful in optical communication systems and to optical devices using such waveplates.
In planar lightwave circuits (PLCs), for example arrayed waveguide gratings (AWGs), polarization dependence is a well known problem. It stems from the fact that the planar waveguides typically exhibit birefringence. The problem is described in detail e.g. in U.S. Pat. No. 5,901,259 issued May 4, 1999 to Ando et al. The specification of the patent is hereby incorporated by reference.
According to the Ando patent, supra, the problem is dealt with by providing an optical waveplate made of polyimide. The waveplate has a specific thickness of 20 xcexcm or smaller. The waveplate is inserted into the optical waveguide circuit so that the waveplate is either perpendicular to or somewhat inclined from the longitudinal direction of the waveguide. The waveguide is typically inserted in the middle of the optical path of the waveguide(s).
The waveplate functions by converting the TE (transverse electric) polarization into TM (transverse magnetic) polarization and vice versa.
While the waveplates of Ando et al. appear to perform their function as intended, their price at this time is relatively high and there is a need to develop a low-cost alternative. Since there are many amorphous polymers in which large in-plane birefringence can be induced, it is not a trivial task to find a polymer that meets stringent reliability requirements for optical communication purposes.
The present inventor has found that a highly effective waveplate can be obtained from properly processed polyethylene naphthalate (PEN). The waveplate must meet basically the same requirements as the polyimide waveplates of the Ando patent, supra.
The waveplate may preferably be in the form of a film having a thickness in the range of 2 to about 25 xcexcm. The film may be stretched uniaxially or biaxially to control its refractive index and thus the birefringence.
The waveplate may form part of an optical waveguide device such as an arrayed waveguide grating.